This invention relates to preparing biopolymers and biopolymer blends by enzyme-mediated catalysis e.g., in a microorganism host. Biopolymers are polymers that can be prepared via enzyme-mediated catalysis either in vitrn or in a microorganism host such as a bacterium. These biopolymers are desirable because they are biodegradable and biocompatible, making them suitable for a number of applications. In a typical procedure, one or more nutrients is fed to bacterial cells containing polymerase enzymes capable of processing the nutrients to form the desired biopolymer. The biopolymer is deposited in the form of osmotically inert, intracellular granules that are then extracted from the cells. This procedure has been used, for example, to prepare homopolymers and random copolymers containing 3-hydroxy organoate units.
The inventors have discovered how to control the microstructure and chemical composition of biopolymers and biopolymer blends produced via enzyme-mediated catalysis. Accordingly, it is now possible to prepare biopolymers and biopolymer blends that are tailored to meet the needs of a specific application. In particular, block copolymers and phase-separated blends can be prepared.
The phase-separated blends feature particles having a biopolymer-containing core substantially surrounded by a biopolymer-containing shell in which the core and shell have different chemical compositions. The biopolymers may be in the form of homopolymers, copolymers, or combinations thereof, where xe2x80x9chomopolymerxe2x80x9d refers to a polymer having a single type of monomer unit and xe2x80x9ccopolymerxe2x80x9d refers to a polymer having two or more different monomer units. The thickness of the shell is less than about 1 micrometer, preferably less than about 0.1 micrometer, and more preferably less than about 0.05 micrometer.
A number of different core/shell combinations can be prepared. According to one embodiment, the core includes a homopolymer and the shell includes a copolymer, preferably in which the copolymer and homopolymer have common monomer units. Alternatively, the core can include the copolymer and the shell can include the homopolymer. The particle can further include one or more additional shell layers.
Particularly useful biopolymers are homopolymers and copolymers that include hydroxy organoate units. Such units have the general formula: 
where R is a group containing between 1 and 30 carbon atoms, inclusive, and n is at least 1. Examples include 3-hydroxy organoate units (n=1) and 4-hydroxy organoate units (n=2). Specific examples include 3-hydroxy butyrate (R=methyl; n=1), 3-hydroxy valerate (R=ethyl; n=1), and combinations thereof. In one embodiment, the core includes a homopolymer containing 3-hydroxy butyrate units and the shell includes a copolymer containing 3-hydroxy butyrate and 3-hydroxy valerate units. In another embodiment, the core includes a copolymer containing 3-hydroxy butyrate and 3-hydroxy valerate units, and the shell includes a homopolymer containing 3-hydroxy butyrate units.
Block copolymers having at least two blocks can also be prepared. Preferably, the blocks contain 3-hydroxy organoate units. For example, the first block may contain 3-hydroxy butyrate units and the second block may contain 3-hydroxy valerate units.
The biopolymers and biopolymer blends may be prepared using microorganism hosts by controlling the nutrients available to the microorganism, resulting in the sequential formation of different polymers, or polymer blocks, within the same granule. In the case of block copolymers, for example, the relative amounts of the nutrients and the timing of introduction into the microorganism may be selected such that the first nutrient is available for reaction substantially throughout the process and the second nutrient is available for reaction throughout selected portions of the process.
The biopolymers and biopolymer blends are useful in a number of applications. For example, they may be compounded with a tackifier and, optionally, a crosslinking agent to form an adhesive composition. Examples of suitable tackifiers and crosslinking agents are described, e.g., in Rutherford et al., U.S. Pat. No. 5,614,576 and Rutherford et al., U.S. Pat. No. 5,753,364, both of which are hereby incorporated by reference. It is also possible to combine the biopolymers and biopolymer blends with another polymer matrix to modify the properties of the polymer matrix. For example, the two phase biopolymer particles can be incorporated into a brittle polymer such as polystyrene to improve the impact resistance of the brittle polymer.
Other features and advantages will be apparent from the following description of preferred embodiments thereof, and from the claims.